controlled release and delivery systems for herbicides are known and can be achieved by:
(a) combining acidic herbicides, e.g., 2,4-dichlorophenoxyacetic acid, with basic polymers, e.g., a polyvinyl-amine, to form herbicide-polymer salts; PA1 (b) utilization of rubbers and elastomers as matrix reservoirs; PA1 (c) use of a herbicide-containing polymer emulsion; PA1 (d) utilization of biodegradable polymers as matrices, e.g., polyureas, polyacetals, cellulose derivatives, polyesters and polyurethanes; PA1 (e) use of synthetic and natural polymeric materials containing covalently-bound pendant herbicide groups; PA1 (f) utilization of inorganic materials such as glass, clays and plaster of Paris as well as organic polymers to incorporate herbicides in pellets and granules; and PA1 (g) encapsulation of herbicides in polymers. U.S. Pat. No. 4,304,769 to Chen discloses "loaded" polymeric latex compositions and their preparation. U.S. Pat. No. 4,303,642 to Kangas discloses stable latexes comprising polymers which contain insecticides. PA1 (a) dissolving the hydrophobic herbicide in a volatile water-immiscible solvent; PA1 (b) introducing the solvent containing said herbicide dissolved therein into a two phase latex dispersion comprising an aqueous continuous phase and a polymeric discrete phase having absorption capacity for said herbicide; PA1 (c) agitating the resultant mixture for a time sufficient for the polymeric particles of the discrete phase to absorb the herbicide therein; and PA1 (d) removing the solvent form the resultant mixture at reduced pressure. PA1 (a) monovinylidene carbocyclic monomers, such as, styrene; styrene ring-substituted by methyl, two methyl groups, ethyl, tert-butyl, cyano, hydroxy, methoxy, ethoxy, or, a methyl and a chloro group; and vinylnaphthylene; PA1 (b) a combination of one of the foregoing monovinylidene carbocyclic monomers with an emulsion-polymerizable co-monomer in the nature of copolymerizable latex stabilizer having from 3 to about 26 carbon atoms; PA1 (c) an ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid having about 3-9 carbon atoms, e.g., methyl methacrylate, chloroethyl methacrylate, n-butyl methacrylate, cyclohexyl chloroacrylate, ethyl chloroacrylate, methyl chloroacrylate, and isopropyl chloroacrylate; and PA1 (d) .alpha.,.beta.-ethylenically unsaturated esters of non-polymerizable carboxylic acids, e.g., vinyl acetate, vinyl propionate, vinyl benzoates optionally carrying a ring substitutent such as lower alkyl of 1 to 4 carbon atoms, lower alkoxy of 1 to 4 carbon atoms, halo, cyano, or hydroxy, e.g., ethyl, vinyl toluate, allyl benzoates, vinyl pivalate and other such monomers wherein the unsaturated group has about 2-14 carbon atoms.
U.S. Pat. No. 3,156,661 to Feinberg discloses the use of styrene latexes as carriers of insecticidal materials. U.S. Pat. No. 4,336,173 to Ugelsad discloses a process for preparing an aqueous emulsion of a partly water-soluble material. Banker and Boylan have reported the use of latexes and pseudolatexes to entrap drugs and insecticides for controlled release. Banker, Abs. of the 7th Int'l. Sym. on Controlled Release of Bioactive Materials, pp. 12-16, 1980, and Boylan and Banker, J. Pharm. Sci., 62, 1177 (1973).
There are several advantages of using a latex for a herbicide controlled delivery system over conventional polymer matrices. First, latexes by their own unique nature exist in stable colloidal form and as such can be easily diluted with water and applied to the soil with conventional spray or aqueous dispersal systems; conventional herbicide containing polymer matrices would have to be converted to some dispersible form, i.e., pellets, granules or an emulsion before they could be applied; none of these physical processes, however, can readily produce small particles equal in size to typical latexes, i.e., about 0.1-1.5 .mu.m, hence the dispersibility of the herbicide in the latex particles is much greater and the resulting greater surface area of the latex particles might afford a better release profile. A second advantage is that commercial latexes can be produced in essentially monodisperse form without the need of further sorting, thereby assuming uniform distribution during application. A third advantage of latex is that many polymers like polyesters, polyamides and polyurethanes are more expensive than some commercial latex polymers and their processes and starting monomers may be more expensive than latex polymers.
A latex controlled delivery system also offers an additional advantage over polymer matrices containing chemically attached pendant herbicide molecules. For example, chemically-attached herbicides, e.g., via ester functions, must undergo hydrolysis to be effective, and the rate of hydrolysis is very much dictated by the extent of moisture in the immediate environment; therefore, the use of chemically-bound herbicide polymers must be confined to environments where the water content is always predictable. In latex systems, after dispersal of the colloidal herbicide containing polymer particles, the presence or absence of environmental water is not critical for release of the herbicide.
The processes of the prior art are disadvantageous, however, when dealing with hydrophobic, hydrolyzable herbicides. Use of a water-miscible solvent as disclosed in U.S. Pat. No. 4,304,769 with a hydrophobic material such as the herbicides contemplated by this invention results in the destabilization of the latex dispersion. When a hydrophobic herbicide material is dissolved in a water-miscible solvent and this solution is brought together with the latex dispersion, the water-miscible solvent blends with the water int eh latex dispersion but this blend of water and the water-miscible solvent is incapable of maintaining the water-insoluble herbicide material in solution. Consequently, the insoluble herbicide material separates form the water/water miscible solvent blend, as an oil, or as a colloid, i.e., in finely dispersed form. For example, when an acetone solution of tridiphane is mixed with water, the tridiphane separates form the resulting water-acetone solution.
Where the herbicide used is subject to substantial hydrolysis over time, it tends to undergo decomposition upon exposure to water, and the use of heat to form the latex dispersion, as taught in U.S. Pat. No. 4,303,642, causes the hydrophobic herbicide to undergo hydrolysis more readily, especially in the absence of any solvent as taught by the reference process.